The polyphenylene ether resins are a family of engineering thermoplastics that are well known to the polymer art. These polymers may be made by a variety of catalytic and non-catalytic processes from the corresponding phenols or reactive derivatives thereof. By way of illustration, certain of the polyphenylene ethers are disclosed in Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875, and in Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358. In the Hay patents, the polyphenylene ethers are prepared by an oxidative coupling reaction comprising passing an oxygen-contained gas through a reaction solution of a phenol and a metal-amine complex catalyst. In the Stamatoff patents, the polyphenylene ethers are produced by reacting the corresponding phenolate ion with an initiator, such as peroxy acid salt, an acid peroxide, a hypohalite, and the like, in the presence of a complexing agent.
Other disclosures relating to processes for preparing olyphenylene ether resins, including graft copolymers of polyphenylene ethers with styrene type compounds, are found in Fox, U.S. Pat. No. 3,356,761; Sumitomo, U.K. Pat. No. 1,291,609; Bussink et al., U.S. Pat. Nos. 3,337,499; Blanchard et al., 3,219,626; Laakso et al., 3,342,892; Borman, 3,344,166; Hori et al., 3,384,619; Faurote et al., 3,440,217; and disclosures relating to metal based catalysts which do not include amines, are known from patents such as Wieden et al., U.S. Pat. Nos. 3,442,885 (copper-amidines); Nakashio et al., 3,573,257 (Metal-alcoholate or -phenolate); Kobayashi et al., 3,455,880 (cobalt chelates); and the like. Disclosures relating to non-catalytic processes, such as oxidation with lead dioxide, silver oxide, etc., are described in Price et al., U.S. Pat. No. 3,382,212. Additional methods of preparing polyphenylene ethers are described in Bennett et al., U.S. Pat. Nos. 3,639,656; Cooper et al., 3,642,699 and 3,661,848, and copending, commonly assigned U.S. patent applications Ser. No. 718,834, filed Aug. 30, 1976, and Ser. No. 718,836, filed Aug. 30, 1976. All of the above-mentioned disclosures are incorporated herein by reference.
The processes most generally used to produce the polyphenylene ether resins involve the self-condensation of a monovalent phenol in the presence of an oxygen-containing gas and a catalyst comprising a metal-amine complex.
At the conclusion of the reaction, the reaction solutions obtained, e.g., by oxidizing 2,6-xylenol with a copper-amine catalyst, are extracted with aqueous mineral acid or acetic acid or a mixture of water and carbon dioxide to remove the metallic component of the catalyst and the amine, before isolation of the polymer of precipitation with an anti-solvent, such as methanol. It is important to remove the metallic catalyst residue from the reaction solution (and the polymer) because contamination of the polymer by metallic residues results in discoloration and degradation.
In Bennett et al., U.S. Pat. No. 3,838,102, a new method is described which is extremely effective for removing metallic residues from polyphenylene ether reaction mixtures. The method yields polymer with very low metal content after precipitation either conventionally by adding an antisolvent or by total isolation procedures. The method of U.S. Pat. No. 3,838,102 involves adding a polyfunctional compound to the reaction mixture, the compound being capable of selectively complexing with the metallic component of the catalyst, to decompose the catalyst complex and to form a water soluble, extractable composition of the metal and the polyfunctional compound.
Molecular weight control problems are also encountered, however. It is known that, when polyphenylene ether reaction mixtures are allowed to stand for appreciable periods before isolation of the polymer, the intrinsic viscosity (I.V.) of the polyphenylene ether is reduced. The extent of the I.V. drop depends on the time between reaction and isolation, the temperature of the mixture, and probably on the conditions used in preparing the polymer. In typical large scale operations, with the reaction mixture held at 50.degree. C., the I.V. drop is usually more than 0.1 dl./g. per hour, and drops greater than 0.2 dl./g. per hour are not uncommon.
In practice, an attempt is made to compensate for this degradation by adjusting the polymerization conditions to prepare a polymer of substantially higher I.V. than that desired in the final product, so that after the I.V. drop between reaction and isolation, the intrinsic viscosity will fall in an acceptable range. However, this method is expensive, because more catalyst is required than would otherwise be necessary, and difficult to control, because the amount of the I.V. drop may vary widely, especially when the interval between the end of the polymerization and precipitation of the polymer is prolonged for any reason. A method for stabilizing the reaction mixtures, that is, preventing or minimizing the I.V. drop in polyphenylene ether reaction mixtures, is, therefore, extremely useful.
German Offen. No. 2,430,130, Jan. 23, 1975, discloses a method for stabilizing the I.V. in polyphenylene ether reaction mixtures by adding a mixture of a dihydric phenol such as hydroquinone or catechol, or a benzoquinone, and a mild reducing agent, such as sodium sulfite. The publication teaches that the dihydric phenol should be used in an amount greater than two moles per gran-atom of the copper or other metal catalyst used in the polymerization, and preferably, at a level of at least 5 moles per gram-atom.
It has been proposed to treat the polyphenylene ether reaction mixture with a combination of (i) a dihydric phenol/reducing agent and (ii) a chelating agent for the metal catalyst, such as a salt of ethylenediaminetetraacetic acid or nitrilotriacetic acid. By such treatment, intrinsic viscosity degradation may be prevented with much smaller amounts of the dihydric phenol than are taught to be necessary in the above-mentioned German publication.
Unexpectedly, it has now been found that certain classes of aromatic amines, used in combination with a metal chelating agent, stabilize the I.V. of polyphenylene ether in polyphenylene ether reaction mixtures.
It is, therefore, a primary object of this invention to control the intrinsic viscosity of phenyphenylene ether in the polyphenylene ether reaction mixture.
It is also an object of this invention to stabilize the molecular weight of polyphenylene ether resin.
Still another object of this invention is to provide a method of preparing polyphenylene ether resin wherein the method results in lower catalyst usage and shorter reaction time.